Remove LRU cache from Alpha (#2837)

The LRU cache in Alpha has been causing a lot of contention. This PR removes the cache altogether, so posting lists are read directly from Badger as needed. Instead of one global LRU cache, each Txn object has a local cache, which is used both for mutations and for queries. Query functions share this local cache.

Pros: We have seen an astounding 5-10x query latency drops due to this PR! I've also seen 1.7x jump in mutation throughput while running the live loader.

Cons: This PR introduces a query local cache, which can potentially cause issues if a query requests for more data than the RAM available in the server. However, that can be dealt with in future PRs.

The `lru_mb` flag has not been removed yet. We intend to use it later when we introduce an LRU cache in Badger.

Changes:
* Remove LRU cache.
* Make query after mutation before commit work. Make tests work.
* Return list from txn.Store
* Fix the query package test failure. Also Txn should always use a local cache.
* No need to commit in memory
* Bring GetNoStore back
* No need to track keys during rolling up.
* Make LocalCache use byte slice instead of string as argument.

Author: manishrjain

dgraph/docker-compose-single.yml

@@ -13,7 +13,7 @@ services:
       - 6080:6080
     environment:
       - GOPATH=$GOPATH
-    command: /gobin/dgraph zero --my=zero1:5080 --replicas 3 --idx 1 --bindall --expose_trace --profile_mode block --block_rate 10 --logtostderr -v=2 --jaeger.collector http://jaeger:14268
+    command: /gobin/dgraph zero --my=zero1:5080 --replicas 3 --idx 1 --bindall --expose_trace --profile_mode block --block_rate 10 --logtostderr -v=2 # --jaeger.collector http://jaeger:14268
     volumes:
       - type: bind
         source: $GOPATH/bin
@@ -35,7 +35,7 @@ services:
       - 9999:9999
     security_opt:
       - seccomp:unconfined
-    command: /gobin/dgraph alpha --my=dg1:7180 --lru_mb=1024 --zero=zero1:5080 --expose_trace -o 100 --trace 1.0 --profile_mode block --block_rate 10 --logtostderr -v=3 --jaeger.collector http://jaeger:14268
+    command: /gobin/dgraph alpha --my=dg1:7180 --lru_mb=1024 --zero=zero1:5080 --expose_trace -o 100 --trace 1.0 --profile_mode block --block_rate 10 --logtostderr -v=3 # --jaeger.collector http://jaeger:14268
     #command: /gobin/dlv --listen=:9999 --headless=true --api-version=2 --init $GOPATH/src/github.com/dgraph-io/dgraph/dgraph/dlv.init exec /gobin/dgraph -- alpha --my=dg1:7180 --lru_mb=1024 --zero=zero1:5080 -o 100 --expose_trace  --trace 1.0 --profile_mode block --block_rate 10 --logtostderr -v=3 --jaeger.collector http://jaeger:14268
 
   jaeger:
@@ -53,34 +53,31 @@ services:
     environment:
       - COLLECTOR_ZIPKIN_HTTP_PORT=9411
 
-  node-exporter:
-    image: quay.io/prometheus/node-exporter
-    container_name: node-exporter
-    pid: "host"
-    volumes:
-      - type: bind
-        source: /
-        target: /host
-        read_only: true
-
-  prometheus:
-    image: prom/prometheus
-    container_name: prometheus
-    hostname: prometheus
-    ports:
-      - "9090:9090"
-    volumes:
-      - type: bind
-        source: $GOPATH/src/github.com/dgraph-io/dgraph/dgraph/prometheus.yml
-        target: /etc/prometheus/prometheus.yml
-        read_only: true
-
-  grafana:
-    image: grafana/grafana
-    container_name: grafana
-    hostname: grafana
-    ports:
-     - "3000:3000"  
+  # node-exporter:
+  #   image: quay.io/prometheus/node-exporter
+  #   container_name: node-exporter
+  #   pid: "host"
+  #   volumes:
+  #     - type: bind
+  #       source: /
+  #       target: /host
+  #       read_only: true
 
-    
+  # prometheus:
+  #   image: prom/prometheus
+  #   container_name: prometheus
+  #   hostname: prometheus
+  #   ports:
+  #     - "9090:9090"
+  #   volumes:
+  #     - type: bind
+  #       source: $GOPATH/src/github.com/dgraph-io/dgraph/dgraph/prometheus.yml
+  #       target: /etc/prometheus/prometheus.yml
+  #       read_only: true
 
+  # grafana:
+  #   image: grafana/grafana
+  #   container_name: grafana
+  #   hostname: grafana
+  #   ports:
+  #    - "3000:3000"